This invention relates generally to the art of manufacturing ribbon cartridges for use with printers for equipment such as computers and word processors. More particularly, the invention provides a method and apparatus for efficiently and accurately adjoining the ends of a length of ribbon so as to form a continuous ribbon loop extending through a ribbon cartridge. Still more particularly, the present invention relates to a printer ribbon welding equipment system that joins two ribbon ends in a cross pattern while controlling the tension of the two ribbon ends at the place of joinder.
Continuous loop printer cartridges are in very common use throughout the world. In simple terms, they comprise a length of nylon, or other fabric ribbon, loaded in serpentine fashion within a cartridge casing and adjoined at the two ends to form a continuous loop. The weakest point along the ribbon loop, and thus the point that typically breaks soonest during use, is the fusion weld which joins the two ribbon ends. Thus, efforts to improve the durability of continuous loop ribbon cartridges concentrate on improvements to the method and apparatus for forming the weld.
U.S. Pat. No. 4,629,530, which is expressly incorporated herein by reference, describes what is presently the most commonly used method and apparatus for joining two ribbon ends to form a continuous loop. The method and apparatus described in this patent is a substantial improvement over the "crash welding" technique previously in common use. Crash welding involves a simultaneous welding and cutting operation which imparts an excessive amount of energy into the fabric ribbon, weakening the ribbon weld and shortening the expected life of the cartridge.
In the improved technique described in U.S. Pat. No. 4,629,530, an operator clamps the two ribbon ends in criss-cross or x-pattern configuration across the top of an anvil having a narrow upper land surface (see FIG. 1 of U.S. Pat. No. 4,629,530). An ultrasonic horn moves into position above the crossed ribbon ends and the anvil, and without crushing the ribbon ends to a point of damage against the anvil, imparts ultrasonic energy into the ribbon ends, fusing the ribbon ends along a line defined by the adjacent land surface of the anvil.
Next, a separate cutter mechanism severs the two ends of the fused ribbon along the edge of the weld line or bead (see FIG. 2 of U.S. Pat. No. 4,629,530), and a mechanism rotates the upper ribbon 180.degree. to give a continuous length of ribbon joined along a diagonal weld bead (see FIG. 3 of U.S. Pat. No. 4,629,530). The weld formed by this technique leaves a distinct nub (see FIG. 4A of U.S. Pat. No. 4,629,530) that is unacceptable in terms of ribbon cartridge performance. Consequently, this technique typically also includes a second welding operation whereby the weld bead on the unfolded ribbon ends is reheated and flattened to reduce the nub to acceptable dimensions.
To achieve maximum weld strength, the ribbon ends must be placed in tension during the weld process. U.S. Pat. Nos. 4,629,530 and 3,821,048 disclose stretching, or tensioning of the ribbon, prior to and during the welding operation. In U.S. Pat. No. 4,629,530, it is noted that the ribbon is held taught by the operator before the clamping apparatus grips the ribbon for welding. Col. 26, lines 10 to 22. In U.S. Pat. No. 3,821,048, the ribbon is tensioned by ribbon tensioning feet which stretch the ribbon into ribbon tensioning cavities as the ribbon is gripped in the ribbon positioning groove for welding. Col. 4, lines 33 to 42. However, there is no disclosure of the need to, or means for, adjusting the tension for different ribbon structures, size or ink formulas.
Maximum weld strength is also affected by the quantity of tensioning, or total tension force. Too much tension can be as detrimental to achieving optimum weld strength as no tensioning whatsoever. Further, the amount of tension force necessary to impart the maximum strength weld in a ribbon varies greatly from ribbon composition to ribbon composition. This variation occurs as a result of variations in the ribbon material, and as a result of variations in the composition of the ink which is often preloaded into the ribbon prior to the welding process.
Different ribbon materials will contain different amounts of nylon, or different grades of nylon, which react differently to the ultrasonic welding process. Likewise, there may be as many as two thousand compositions of ink which may be used with cartridge ribbons. Therefore, the combinations of ribbon and ink result in a great variation of ribbon welding properties. The proper welding tension for each ribbon or ribbon and ink combination is usually determined by trial and error, by welding the ribbon at different tensions and then testing the weld strength to obtain an optimum weld strength tension. However, as the tension imparted during the welding operation is in large part based upon a given operator's feel for tension, the optimum tension is commonly not readily reproducible from operator to operator, and each operator must determine the feel required to impart the optimum tension. Further, as the operator's shift progresses, or when the operator resumes operation after a break or day or days off from the operation, the requisite feel may change leading to non-optimum welding tension.
Thus, it is apparent that it would be advantageous to develop a method and apparatus for controlling ribbon tension during welding which is independent of the operator's perception of the proper tension.